1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and electrophotographic apparatus.
2. Description of the Related Art
An electrophotographic photosensitive member which includes a photoconductive layer (photosensitive layer) made of an amorphous material on a substrate is well known, and in particular, electrophotographic photosensitive members which include a photoconductive layer of hydrogenated amorphous silicon (hereinafter also referred to as “a-Si:H”) formed on a metal substrate by a layer formation technique such as CVD or PVD have already been introduced commercially. Hereinafter, an electrophotographic photosensitive member may be referred to simply as a “photosensitive member.” Also, an electrophotographic photosensitive member provided with photoconductive layer made of a-Si:H may be referred to as “a-Si:H photosensitive member.” Furthermore, a photoconductive layer made of a-Si:H may be referred to as an “a-Si:H photoconductive layer.” A basic configuration of such an a-Si:H photosensitive member 4000 includes an a-Si:H photoconductive layer 4002 formed on a conductive substrate 4001 and a surface layer 4003 formed on the photoconductive layer 4002, as shown in FIG. 4. The surface layer 4003 contains hydrogenated amorphous silicon carbide (hereinafter also referred to as “a-SiC:H”). Hereinafter, a surface layer made of a-SiC:H may be referred to as an “a-SiC:H surface layer.
The surface layer 4003 is an important layer which has a bearing on electrophotographic characteristics. Characteristics required of the surface layer include wear resistance, moisture resistance, charge retention, and light transmission. Surface layer made of a-SiC:H excel especially in wear resistance and offer a good balance among the above-mentioned characteristics, and thus have been used mainly for electrophotographic apparatuses with high processing speed. However, conventional surface layer made of a-SiC:H could cause image deletion (hereinafter also referred to as “high-humidity image deletion”) when used in high-humidity environment.
The high-humidity image deletion is an image defect which occurs in an electrophotographic process when image formation is repeated in a high-humidity environment and images are output again after a while and in which characters become blurred or characters fail to be printed. This phenomenon is caused in part by moisture adsorbed on a surface of the photosensitive member. To prevent occurrence of high-humidity image deletion, it is conventional practice to constantly heat the electrophotographic photosensitive member by a photosensitive-member heater, thereby reducing or removing the moisture adsorbed on the surface of the photosensitive member.
On the other hand, techniques have conventionally been proposed which prevent high-humidity image deletion without using a photosensitive-member heater. Japanese Patent No. 3124841 describes a technique for forming an a-SiC:H surface layer in an a-Si:H photosensitive member, which is made up of a photoconductive layer and the a-SiC:H surface layer formed in sequence on a substrate, wherein atom densities of silicon atoms, carbon atoms, and hydrogen or fluorine atoms in the a-SiC:H surface layer are reduced below predetermined values. The technique disclosed in Japanese Patent No. 3124841 gives a relatively coarse layer structure to the a-SiC:H surface layer by reducing the atom density of each atom in the a-SiC:H surface layer below the predetermined values, thereby allowing the surface layer to be scraped easily in a cleaning process. Consequently, it is stated that a new surface with reduced moisture absorption is always obtained, thereby allowing prevention of high-humidity image deletion.
On the other hand, from the viewpoint of charge retention, an attempt to improve an a-SiC:H surface layer has been proposed. Japanese Patent Publication No. H5-018471 proposes an a-Si:H photosensitive member made up of an a-Si:H photoconductive layer and two a-SiC:H surface layers formed in sequence on a substrate. With the technique disclosed in Japanese Patent Publication No. H5-018471, the outer of the two a-SiC:H surface layers has a higher defect density than the surface layer on the side of the photoconductive layer. Japanese Patent Publication No. H5-018471 states that the increased defect density in the outer surface layer enables forming a layer thickness needed to ensure wear resistance while improving charge mobility and preventing increase in residual potential. Also, Japanese Patent Publication No. H5-018471 states that the decreased defect density in the surface layer on the side of the photoconductive layer enables ensuring charge retention.
Recently, there has been demand to meet the needs for higher speed, higher image quality, and longer lives in electrophotographic processes while at the same time achieving power savings from the viewpoint of environmental friendliness. From this point of view, the photosensitive member is expected to be improved further. For example, regarding moisture resistance, image quality is required to be increased because high-humidity image deletion can cause deterioration of image quality. If the photosensitive-member heater is installed to prevent high-humidity image deletion, a considerable amount of standby power is required even when the electrophotographic apparatus is not running. Also, with the technique disclosed in Japanese Patent No. 3124841, a surface of the electrophotographic photosensitive member needs to be worn at a certain level of speed, and thus durability tends to be lost especially in a high-speed electrophotographic process. Possible causes of the durability loss include pressure scars and flaking as well as surface wear.
The pressure scars is a phenomenon in which image defects such as black streaks or white streaks appear on an image when mechanical stresses are applied to the electrophotographic photosensitive member. The pressure scars hardly occurs in normal use of the electrophotographic photosensitive member, but can occur on rare occasions when foreign matter is contained in printing paper. The pressure scars tends to stand out in a high-definition electrophotographic process especially when a halftone image is output. Thus, once it occurs, the pressure scars will reduce image quality and can result in shortening the life of the electrophotographic photosensitive member. The flaking is a phenomenon in which part of a surface layer flakes off. Once flaking occurs in an image forming area of the electrophotographic photosensitive member, it is difficult to continue using the electrophotographic photosensitive member. There is demand to satisfy durability and light transmittance at a higher level so as to support the latest electrophotographic processes while ensuring these properties, assuming a configuration in which no heater is used. Some of these properties are improved individually by the techniques described in Japanese Patent No. 3124841 and Japanese Patent Publication No. H5-018471. However, neither Japanese Patent No. 3124841 nor Japanese Patent Publication No. H5-018471 makes any technical suggestion on how to satisfy these properties at a higher level.